Oenothera Lamarckiana and O . gigas. 947 
spireme appears (Fig. 13), which is so short that the band may be followed 
distinctly in its convolutions throughout the nuclear cavity. The spireme 
is in some cases unquestionably a single thread, although the loops are 
frequently joined by delicate cross strands, and it is sometimes attenuated 
at points. In other cases it appears as though the spireme is not strictly 
continuous, but consists of two or more somewhat distinct portions. 
Following the differentiation of the spireme (Figs. 12 and 13), a process 
of segmentation begins (Figs. 14 and 15) which transforms the spireme 
into a chain of fourteen chromosomes. This segmentation consists in 
a drawing apart of certain regions of the spireme as though by a process 
of constriction (Fig. 14), or perhaps more accurately by the condensation of 
the chromatin at fourteen chromosome centres on the spireme. This con¬ 
densation results in a still further shortening of the spireme so that the 
complexities of its early looped arrangement largely disappear, and the 
chromosome segments may be followed for long distances as parts of a chain 
(Figs. 15, 1 6, and 17). The chain is sometimes broken as though a few 
segments had been displaced, which might readily result from such a looping 
of the spireme that parts would be thrown out of the general line of arrange¬ 
ment, or because portions of the original spireme were separated at points. 
During the segmentation of the spireme and the condensation of its 
portions to form the fourteen sporophytic (somatic) chromosomes the 
segments are frequently found grouped at one side of the nucleus in a close 
mass (Figs. 18-20) that resembles superficially the chromatic contraction 
of synapsis. This condition is more conspicuous in Lamarckiana than in 
my material of biennis , and constitutes a very clearly marked stage in the 
reduction processes which will be described as the second contraction, since 
it appears to correspond closely in the time of its appearance with the 
c second contraction 5 stage of a number of writers on different types. This 
stage in Oenothera takes place during the segmentation of the spireme or 
shortly after, so that the contracted material consists of chromosome 
segments either in process of formation (Figs. 18 and 19) or in a somewhat 
more advanced stage in their condensation into chromosomes (Figs. 20 
and 21). 
Gates (’ 08 , pp. n and 12) places the stage of ‘second contraction’ in 
rtibrinervis previous to the segmentation of the spireme, but his figures are 
not consistent with his account. Thus Fig. 20 exhibits a pair of chromo¬ 
somes cut off from the remainder of the chromatin, which, in spite of the 
lack of detail shown, has the appearance of a contracted, looped, and 
segmented spireme, or perhaps even of a chain of chromosomes. Fig. ai, 
with an even greater lack of detail present in the drawing of the contracted 
chromatin, shows evidence of a segmenting spireme in the loop extending 
into the nuclear cavity. Fig. 18 may illustrate, as described, the shortened 
and thickened thread (spireme) entering upon the phase of second con- 
